Studies of gonads52,73, blood types7,31,50, chromosomes7,41 and red cell antigens (ABO, Rh, M, N, S, P1, K, Fy and Jk)31 have shown the fetus-in-fetu to be monozygotic to the host. It is not surprising, however, that the fetus-in-fetu has the same blood type as the host, since it is perfused by the host. There have been no recorded cases of dizygosity. However, as in acardiac twins, the combination of a normal twin with a twin that has lost a gonosomic chromosome and thus appears as a 45,X0 could potentially be found.
At surgery, the fetus-in-fetu appears as a well-circumscribed mass bound by a fibrous membrane. Inside the mass the fetus-in-fetu is suspended in straw-colored fluid by a pedicle. Two vessels (an artery and a vein) travel along the pedicle. The fluid is generally not abundant and has been described as containing sebaceous material. The origin of this fluid is uncertain. Several authors have pointed out that the membranes of a normal embryo are not responsible for the production of amniotic fluid. They would more likely act as semipermeable membrane. In fetuses past 12 weeks, the urinary system produces the fluid and the gastrointestinal system resorbs it. Since no fetus-in-fetu has ever been described to contain a urinary system, and the segments of gastrointestinal tract that are found are too incomplete to have any reabsorptive capabilities, the fluid is probably maintained in the amniotic cavity solely by osmotic and oncotic pressure.
The presence of chorionic villi has only been reported in one case49. Except for one quote of a 15-year old-saying "Mother, do come to me, I have something alive in my bodyâ€, and the mother being quoted as saying that she felt something resembling "the motion of a child during gestationâ€, no fetal movements have ever been recorded in a fetus-in-fetu. The above record of movement is somewhat doubtful since striated muscles have rarely been found around joints. Yet this host is one of the older hosts described (fig. 7).
What is included
Fetuses in fetu resemble poorly formed acardiac twins. Most every organ has been recognized in various stages of development. The notable exception is the urinary tract, which does not appear to have been recognized in any of the cases that we reviewed.
Some structures such as ribs7, intrathoracic organs (lung12, heart, thymus57) and retroperitoneal organs (liver, spleen, kidneys, adrenal glands34, pancreas34,57, gonads11,34,52,73) are rarely described. An incomplete heart has been found7,22,57,61-62. In one instance, a rudimentary 2-chamber heart was found with the atrium in the caudal position and the ventricle in the cranial position7. This is the stage normally reached in a 22-day embryo67. Facial and cranial structures also are uncommonly seen, yet eyes34, ears, mouth and poorly organized brain and cerebellum have been observed12,20,23,29,31,34, 50,57,65.
The cord that connects the fetus to the membrane has different characteristics than a normal cord: it contains vasa vasorum and nerve fibers.
The evolution of the fetus-in-fetu is usually arrested at the first trimester, and further evolution is by mass accretion more than by development. Overall structures derived from the ectoderm are better represented than structures derived from the other two layers. The mesoderm contributes the musculoskeletal system which is usually well represented, but the other derivatives (the vascular and urogenital system, the spleen and adrenal glands) are uncommonly found. The most commonly represented derivative from the endodermal layer is the gastrointestinal tract, but the liver and pancreas are also often recognized.
The few cases detected prenatally all presented as a complex mass. The general appearance is a well-delineated capsule, with an echogenic mass suspended in fluid or partially surrounded by fluid. Occasionally, the diagnosis can be suggested by the recognition of a rudimentary spine, as in the present case.
When discovered in a newborn child during physical examination, the differential diagnosis includes all the common masses such as Wilms" tumor, hydronephrosis, and neuroblastomas. Prenatally, the main differential diagnosis is with teratoma. Teratomas are disorganized congregations of pluripotential cells from all three primitive tissue layers. By differentiation and induction, they can achieve striking organization, with examples of several organs being well formed. However, teratomas do not have vertebral segmentation, craniocaudal and lateral differentiation, body coelom or systemic organogenesis. Thus the presence of a mass with a spinal organization and surrounded by fluid suggests the correct diagnosis.
When spinal structures are not present, most authors have considered that the diagnosis of fetus-in-fetu can still be made when the alternate criteria described under Definition are found56. These criteria are sufficiently restrictive that even well-organized teratomas cannot fulfill all of them.
Teratomas have a definite malignant potential, a feature that has not been reported in fetus-in-fetu. Teratomas occur predominantly in the lower abdomen, not the upper retroperitoneum. Yet, the coexistence of a fetus-in-fetu and a teratoma6,35,51 as well as the occurrence of a teratoma40 14 years after removal of a twin fetus-in-fetu42 have been reported, supporting the older hypothesis of a continuum between twin and teratoma. Cases of sacrococcygeal fetus-in-fetu should probably be regarded and treated as teratoma, because of the high incidence of teratoma in this region58,68.
Ectopic testicles have a higher incidence of germ cell tumors72, and the differentiation between fetus-in-fetu and teratoma is particularly important. Although the characteristics of intracranial teratoma differ from those of intracranial fetus-in-fetu, Wakai found, in a large review of 245 intracranial teratomas, that there are some transitions between certain teratoma and fetus-in-fetu63.
In the older literature, several descriptions of fetus-in-fetu were too vague to be acceptable by current criteria. For example, the case reported by Phillips does not unequivocally suggest the criteria described above and therefore should probably be considered a teratoma17.
Some have argued that fetus-in-fetu should be considered as teratomas since they do not evolve into lithopedion like fetuses of abdominocyesis3. That argument is probably not valid since in abdominocyesis the antigen complements of the host and fetus are different, which contrasts with fetuses in fetu.
Every organ of the fetus-in-fetu has undergone hypoxic growth and is deformed. Most cases are anencephalic. Usually the body is closed, but ventral wall defects such as omphalocele are common, and a case that suggests a limb-body wall complex has also been described22. The host rarely presents any anomalies, except those related to the presence of a space-occupying lesion. Those manifestations have rarely been severe, even in the case of intracranial fetus-in-fetu28, although in rare cases, severe hydrocephalus was responsible for the death of the host22, 23. One case of Meckel diverticulum and another of skin hemangioma have been described. A malignant degeneration has never been reported, even in the cases that have been allowed to evolve for several years.
Left alone, fetus-in-fetu do not seem harmful to the host, but in every case in which the fetus-in-fetu was not removed at the time of discovery, a slow growth has been described21,38,44.
In the literature of the past century, fetus-in-fetu was fatal to the host because of the compression imposed to adjacent organs15-17. In the more recent literature, the outcome for the host twin is usually favorable. Only a few cases of spontaneous12or postsurgical deaths are recorded7,22,32,44.
There is no report of recurrence.
Aside from a few attempts, in the first half of this century36,73 to marsupialize fetus-in-fetu, surgical removal is the treatment of choice. The membranous capsule can usually be enucleated from the host with minimal problems. In only a few cases, removal is difficult due to adhesions21,37,38, and this difficulty may precipitate the end of the operation36,53 or even be the reason of the postoperative death of the host44. Leaving the capsule, or part of it, has not led to complications, except in very rare cases in which fluid reaccumulated in it36.
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